1. Field of the Invention
The present invention relates to a substrate for a solar cell, a method for producing the same, and a substrate treatment apparatus. In particular, the present invention relates to unevenness of the surface of a substrate for a thin film solar cell and a treatment process for forming the unevenness.
Furthermore, the present invention relates to a thin film solar cell and a method for producing the same. In particular, the present invention relates to unevenness of the surface of a substrate for a thin film solar cell and a method for forming the unevenness.
2. Description of the Related Art
Amorphous silicon (Si) solar cells can be produced at a relatively low temperature of around 100.degree. C. to 200.degree. C. Therefore, various materials can be used for a substrate for forming an amorphous Si solar cell. In general, a glass substrate and a stainless steel substrate are used often.
In the amorphous Si solar cell, the thickness of a light absorbing layer at which a conversion efficiency becomes maximum is as thin as around 500 nm. Thus, in order to improve the conversion-efficiency, it is important to increase the amount of light absorbed by the light absorbing layer having a thickness in this range. Because of this, conventional solutions have used a transparent conductive film with unevenness on its surface formed on a glass substrate or a metal film with unevenness on its surface formed on a stainless steel substrate to increase an optical path length of light in the light absorbing layer.
In the case of increasing an optical path length in the light absorbing layer by the above-mentioned methods, a short-circuit current is known to improve by 30% or more, compared with the case where an amorphous Si solar cell is formed on a flat substrate without unevenness.
In order to form unevenness on the surface of a glass substrate, unevenness is generally formed on an SnO.sub.2 film provided as a transparent electrode by an ordinary pressure CVD method. Such methods for forming unevenness on transparent electrodes are disclosed in Japanese Laid-Open Patent Publication Nos. 58-57756 and 59-159574. However, these publications refer to only the average sizes of the crystal particles on the transparent electrodes and fail to describe the magnitude of unevenness formed on the transparent electrodes.
Furthermore, in order to form unevenness on the surface of a metal substrate made of stainless steel or the like, an Ag film is formed by vapor deposition or sputtering in such a manner that the Ag film is provided with unevenness by adjusting the forming conditions of the Ag film, or an Ag film is formed and heat-treated so as to have unevenness.
In contrast, Japanese Laid-Open Patent Publication No. 2-164077 discloses that unevenness is directly formed on the surface of a glass substrate and the substrate thus provided with unevenness is used for a solar cell. Japanese Laid-Open Patent Publication No. 7-122764 discloses a method for treating the surface of a glass substrate for a solar cell by sandblasting, using abrasive grains with a count of #100 to #8000, in particular #200, thereby forming a scattering reflective surface having unevenness whose average difference in height between convex portions and concave portions is 3 .mu.m.
Japanese Laid-Open Patent Publication No. 1-219043 discloses a method for obtaining uniform unevenness of the surface of a substrate. According to this method, minute and uniform unevenness is formed on the surface of a glass substrate by sandblasting, and then, a SnO.sub.2 film containing large crystal particles with unevenness whose average difference in height between convex portions and concave portions is 0.05 to 0.5 .mu.m is formed on the glass substrate. However, this method does not utilize the unevenness directly formed on the glass substrate by sandblasting. The glass substrate is first subjected to sandblasting so as to have unevenness, and the SnO.sub.2 film is formed thereon. Thus, in this method, the SnO.sub.2 film with more uniform unevenness can be formed, compared with the conventional example in which a SnO.sub.2 film with unevenness is formed on a flat glass substrate. Referring to the description of the specification and drawings of Japanese Laid-Open Patent Publication No. 1-219043, the unevenness on the glass substrate is judged to be smaller than that of the SnO.sub.2 film. In this method, relatively large abrasive grains with a count of #2000 or lower are used for forming unevenness on the glass substrate.
A SnO.sub.2 film can be formed on a glass substrate by an ordinary pressure CVD method, which is easily conducted. However, a temperature for forming the SnO.sub.2 film is required to be high, i.e., around 500.degree. C., so that the SnO.sub.2 film cannot be formed on reinforced glass. This is because reinforced glass has its reinforcement degraded at a temperature of 300.degree. C. or higher. However, reinforced glass is required for protecting the surface of a power solar cell.
In the case of using an ordinary glass substrate which is not reinforced, there is a problem that a solar cell module has a double-structure of reinforced glass and ordinary glass, resulting in the increase in production cost.
Furthermore, in order to sufficiently scatter light, the transparent electrode as described above is required to be formed to a thickness of around 1 .mu.m. This causes an increase in time required for forming a substrate for a solar cell and material cost.
Conventionally, there has been a method for directly forming unevenness on the surface of a glass substrate. However, according to this method, unevenness as minute as that obtained by using a transparent conductive film cannot be formed. For example, Japanese Laid-Open Patent Publication No. 2-164077 discloses that unevenness is formed on the surface of a glass substrate by treating the substrate, using a machine grinding method. According to this method, since the magnitude of the unevenness depends upon the size of a blade used for grinding, the difference in height between convex portions and concave portions becomes tens of .mu.m. Furthermore, there are problems because it takes a long time to form the unevenness and the unevenness having a magnitude most effective for improving efficiency, i.e., minute unevenness with a thickness equal to or smaller than an amorphous film, cannot be formed.
In the case where a stainless steel substrate is used for a solar cell, unevenness is formed on the surface of the stainless steel substrate by adjusting the conditions of forming an Ag film by vapor deposition or sputtering, or by conducting a heat treatment after forming an Ag film. An Ag film with unevenness is required to be formed at a high temperature of 350.degree. C. or higher. Therefore, there are problems because it takes a long time to increase and decrease the temperature of the substrate and the substrate warps. Furthermore, since stainless steel substrates have scratches on their surfaces, it is required to subject the substrate to a grinding treatment for the purpose of improving yield, which results in the increase in cost.
According to the above-mentioned method disclosed in Japanese Laid-Open Patent Publication No. 7-122764, since the magnitude of unevenness of the surface of a substrate formed by sandblasting is large, a defective layer caused by sandblasting is required to be removed by etching. Furthermore, according to this method, in order to smooth the concave portions of the unevenness, SiO.sub.2 is required to be deposited in the concave portions. In the sandblasting, liquid in which abrasive grains are dispersed in water is sprayed onto the surface of a substrate. Therefore, acute unevenness is formed, depending upon the count of the abrasive grains.
According to the above-mentioned method disclosed by Japanese Laid-Open Patent Publication No. 1-219043, there is a problem that the steps of sandblasting, cleaning, and drying are added to the conventional process, resulting in an increase in production cost.
As described above, according to the conventional methods, in the case where a transparent conductive film or an Ag film is formed on a substrate, a high temperature of 350.degree. or higher is required for forming unevenness on the surface of the transparent conductive film or the Ag film. Furthermore, a heat treatment after the formation of the transparent conductive film or the Ag film requires a temperature of around 500.degree. C. Because of this, it takes a long time to increase and decrease the temperature during a treatment at high temperature, which results in low throughput of an apparatus for producing a substrate for a solar cell and large power consumption.